1. Field of the Invention
The present invention relates to a polyvinyl acetal, and to an interlayer film for laminated glass, a binder for ceramic forming, a binder for ink or paint and a thermally-processable photographic material that comprise it.
2. Description of the Related Art
It has been known since long ago that polyvinyl acetal is obtained by acetalyzing polyvinyl alcohol with an aldehyde compound under an acidic condition. Polyvinyl alcohol generally has vinyl alcohol units and vinyl ester units. Therefore, polyvinyl acetal obtained through acetalization of such polyvinyl alcohol comprises at least three types of monomer units that include vinyl acetal units in addition to the two types of monomer units of the starting polyvinyl alcohol. Recently, many different types of polyvinyl alcohol have been proposed, and many different types of polyvinyl acetal have been known by combining such different types of polyvinyl alcohol with various types of aldehyde.
Above all, polyvinyl formal to be produced from polyvinyl alcohol and formaldehyde, polyvinyl acetal to be produced from polyvinyl alcohol and acetaldehyde, and polyvinyl butyral to be produced from polyvinyl alcohol and butylaldehyde stand the essential situation in trade.
In particular, polyvinyl butyral is used not only for interlayer films for windowpanes for automobiles and buildings but also in other various industrial fields of binder for ceramic forming, photosensitive materials, dispersant for ink, etc.
Polyvinyl butyral has a hydrophilic hydroxyl group and a hydrophobic butyl acetal group and is therefore hydrophilic and hydrophobic, and it is much used in various fields as above. In particular, the hydroxyl group in the molecule of polyvinyl butyral plays an important role, for example, in the adhesiveness of the polymer to glass and in the binder force thereof to ceramics, but, on the other hand, it causes some problems in that it readily absorbs moisture in air and, when films formed of polyvinyl butyral are stored for a long term, the plasticizer in the films may readily bleed out. Therefore, the polymer is desired to overcome the problems and to satisfy the requirements for it. Specifically, polyvinyl butyral is desired to have improved waterproofness and improved compatibility with plasticizer, still having the advantages intrinsic to it.
Polyvinyl butyral is widely used as interlayer films in laminate glass for windowpanes for automobiles and buildings. These days, however, the demand for further improving the properties of laminate glass is increasing more and more. For example, when laminate glass is exposed to high humidity for a long time, it may face some problems in that water may penetrate into it through its edges and it may whiten as its compatibility with plasticizer is not good. To solve these problems, various proposals have been made. For example, special silicone oil is additionally used (as in JP-A 7-314609), or triethylene glycol di-2-ethylhexanoate is used as a plasticizer (as in WO00/18698). However, the plasticizer, triethylene glycol di-2-ethylhexanoate is problematic in that its application to conventional polyvinyl butyral is limited to only some particular compositions, as so disclosed in JP-T 2001-515527 (the term “JP-T” as used herein means a published Japanese translation of a PCT application).
Using polyvinyl acetal modified with an α-olefin for interlayer films for laminate glass is proposed in, for example, JP-A 63-79741. On the technical level in those days, however, no one has taken the above-mentioned problems into consideration and, at present, therefore, no one has succeeded in solving the problems.
In the field of binder for ceramic forming, polyvinyl acetal is used, for example, as a forming binder in the process of producing ceramic laminate capacitors or producing ceramic substrates for electronic circuits. In particular, polyvinyl acetal is much used as a binder in producing ceramic green sheets.
These days, in particular, precision electric appliances such as mobile phones and notebook-size personal computers are desired to be small-sized and lightweight, and small-sized, electric and electronic parts with a good performance for these are desired.
For example, small-sized, large-capacity ceramic laminate capacitors are desired, for which various methods have been tried for more reducing the thickness of the electrode part or the ceramic part and for further increasing the capacity. From the technical viewpoint, it is a matter of great importance to thin the ceramic green sheets for them. For reducing the thickness of the sheets, ceramic powder having a small particle size must be used as the starting material. However, ceramic powder having such a reduced particle size shall have an increased surface area and will therefore readily aggregate. Accordingly, the ceramic powder of the type causes some problems in that the surface of the ceramic green sheets formed of it is often roughened, homogeneous ceramic green sheets are difficult to obtain, and the mechanical strength of thin sheets is low.
These problems are considered especially serious for small-sized and lightweight electric and electronic appliances that are desired these days. For example, JP-A 63-79752 discloses some examples of α-olefin-modified polyvinyl acetal. On the technical level in those days, however, no one has taken the above-mentioned problems into consideration and, at present, therefore, no one has succeeded in solving the problems.
In the paint field of its industrial applications, polyvinyl acetal is used for paint for automobiles, baking enamel, shop primer, wash primer, adhesive lacquer, insulating coat on tar or nicotine, paint for plastics, nitrocellulose lacquer, paper varnish, etc. For binder for printing ink for wrapping or packaging materials, used is polyvinyl butyral of low solution viscosity. The printing ink that comprises the polymer well adheres to organic and inorganic substrates and is therefore suitable for prints on polyolefin films, metal foil, cellulose acetate films, polyamide films and polystyrene films.
These days, in particular, printers are driven at high speed in many cases. Therefore, for realizing such high-speed driving of printers, it is said that the printing ink must have a high pigment content at its desired viscosity, and, even if the printed ink film is thin, the print color intensity must be high. In general, for increasing the pigment content of printing ink, it is important to lower the solution viscosity thereof. For lowering the solution viscosity of printing ink, use of polyvinyl acetal having a low degree of polymerization may be taken into consideration. However, using polyvinyl acetal of low polymerization, which is produced through acetalization of completely hydrolyzed polyvinyl alcohol, is problematic in that the aqueous solution of the polyvinyl acetal readily gels and the pigment content of ink with the polymer could not increase.
To solve these problems, for example, proposed are a method of using polyvinyl butyral produced from polyvinyl alcohol having a specific degree of hydrolysis (as in JP-A 11-349889), and a method of using polyvinyl acetal produced from polyvinyl alcohol having 1-alkylvinyl alcohol units and 1-alkylvinyl acetate units (as in JP-T 2000-503341). These methods may be effective in some degree for solving the problems as above, but could not always lead to satisfactory results.
Thermally-developable photographic materials are now in practical use for X-ray photographs in the field of medical therapy, since they may be simply processed for development not producing any excess chemical wastes, as compared with conventional, wet-processable silver halide photographic materials. Such thermally-developable photographic materials may be fabricated by applying an organic silver salt, a reducing agent and a silver halide that is kept in catalytic contact with organic silver ions, on a support such as plastic film by the use of a film-forming binder such as polyvinyl acetal.
The thermally-developable photographic materials of the type have some problems in that the storage stability of the coating liquids to be used in fabricating them is often poor, the sensitivity of the photographic materials themselves is often not so good and the storage stability of the images formed after development is often poor. To solve these problems, for example, a method of using polyvinyl acetal that has a specific ionic group (JP-A 2001-222089) and a method of combining two different types of polyvinyl acetal each having a specific degree of polymerization (JP-A 2002-201215) have been proposed. At present, however, these method could not satisfactorily solve the problems as above.